1. Field of the Invention
The present invention relates to a digital camera that is capable of continuously shooting still images.
2. Description of the Related Art
A digital camera capable of continuously shooting still images at a frame rate of 10 per second is known. With this type of digital camera, an AF (auto focus) operation for a photographing lens is carried out before commencing continuous shooting, and continuous shooting is started after achieving focus of the photographing lens. However, there are cases where the photographing lens goes out of focus during the continuous shooting. A digital single lens reflex camera that carries out an alarm display on display means in the event that it has been determined that a contrast value during a continuous shooting operation is lower than a contrast value immediately after shooting is disclosed in Japanese patent laid-open No. 2005-24858 (laid open Jan. 27, 2005) (hereafter referred to as patent document 1).
With the digital single lens reflex camera disclosed in patent document 1, taken images are stored during continuous shooting, stored images equivalent to an AF target are read out after the storing of the taken images to calculate contrast, and a focused state is determined based on this contrast. It is therefore possible to use in determination of lowered contrast, but it is not possible to use as information for focusing.
The reason why image data during continuous shooting can not be used as information for focusing with this related art digital camera will be described using FIG. 15. An image sensor 1 converts a subject image to image data under control of a TG (Timing Generator) 3, and outputs the image data. At the time of live view display the TG 3 outputs image data to the extent of the VGA standard (640×480 pixels) for live view display to a DRAM 5. Image data temporarily stored in the DRAM 5 is subjected to image processing for live view display by an image processing section 17, and then output to a TFT (TFT liquid crystal display) that acts as a display, and live view display is carried out. Also, image data that has been output from the image sensor 1 has a contrast value calculated as an AF evaluation value by an AF evaluation value calculating section 15, and automatic focusing of the photographing lens is carried out based on this contrast value.
On the other hand, the TG 3 outputs image data for all pixels to the DRAM 5 as taken image data for storage. Image data that has been temporarily stored in the DRAM 5 is image processed for storage by the image processing section 17, and subjected to image compression by a compression processing and image file making section 19, followed by being output to storage media 9 and image data storage being carried out. Recent digital cameras and the like demand high resolution, with the number of pixels of images sensors increasing, and image data output from an image sensor 1 at the time of still image shooting corresponds to all pixels, which means that the data amount becomes extremely large.
In a case where an AF evaluation calculation section 15 for processing image data corresponding to all pixels output from the image sensor 1 at the time of this type of still picture shooting is constructed in hardware, from a viewpoint of high speed processing the technological hurdles are high, which leads to a cost increase. On the other hand, in the case where an AF evaluation value is calculated using firmware processing, processing time becomes long, which is impractical from the viewpoint of high speed continuous shooting. At the time of continuous shooting etc. therefore, as described previously, it is possible to determine whether or not contrast has lowered, but it is not possible to use as information for focusing. It has not been possible to improve following capability of the focusing operation during a continuous shooting operation.